The present invention relates in general to visual inspection apparatus and, in particular, to image detection technology for use with such apparatus. More particularly but not limited to, the invention relates to image pickup devices for use in detecting a two-dimensional (2D) image of an object to be sensed through the main scanning (H scanning) and subsidiary scanning (V scanning) operations.
One typical approach to detection of a 2D image using a linear image sensor is to read image data out of the linear image sensor while simultaneously permitting a projection image of an object being sensed on the image formation plane to move in a direction perpendicular to the linear image sensor, wherein the internal scanning (H-scanning) gets started every time the projected object image moves a distance corresponding to a single pixel (V-scanning). This H-scan distance may be the size of one pixel in the V direction.
For detection of a movement amount (V-scanning) of the object image in this case, a scale for use in detecting the coordinate system of a workpiece mount table is employable, by way of example.
Currently available scale and its associative detection scheme for detecting the movement amount or coordinates (position) of an object being sensed may include a variety of types of techniques based on various principles, such as the transmitted light detection scheme, diffraction light interference scheme and the like. In any one of these approaches, a graduation detection signal is of a sinusoidal waveform having two phases—phase “A” and phase “B”—with the phase difference of 90°, the sine-wave signal being wave-shaped into a pulse signal for use as a coordinate signal.
In addition, in order to obtain a higher degree of resolution than the graduation of scale, a certain scheme is widely employed which employs a divider circuit for dividing the period of a graduation detection signal into several tens of segments or more.
The term “scale resolution” as will be used in the description below refers to the resulting degree of resolution after execution of the period division processing, rather than the resolution of the scale per se.
It is also noted that the term “scale detection pulse” as used herein refers to a pulse signal obtainable after execution of the division processing in cases where the period is divided.
Conventionally, setup of a pixel size along the movement (in the V direction) of the object to be sensed is such that the scale detection pulse is such that the scale detection pulse is frequency-divided at a predefined frequency division ratio to provide an internal scan start signal of a linear image sensor used. This is because the linear image sensor is internally scanned at equal distances or intervals each equivalent to the pixel size.
Note that one prior known technique for variable control of the accumulation time length in order to control the detection sensitivity of a linear image sensor is disclosed in JP-A-62-225081.
Another technique is disclosed in JP-A-6-133209, which is to deal with CCD shift clocks independently of each other at the upper and lower ends of a known time delay and integration (TDI) image sensor in the case where an image within a pickup view filed is different in movement speed or rate between the upper and lower ends, thereby equalizing them to respective image movement rate values.